The relevant standards ITU-T Rec. X.667, ISO/IEC
9834-8:2005, and RFC 4122 specify four algorithms for
generating UUIDs, identified by the version numbers 1, 3, 4,
and 5. (There is no version 2 algorithm.) Each of these
algorithms could be suitable for a different set of
applications.

Table F-33. Functions for UUID Generation

Function

Description

uuid_generate_v1()

This function generates a version 1 UUID. This
involves the MAC address of the computer and a time
stamp. Note that UUIDs of this kind reveal the
identity of the computer that created the identifier
and the time at which it did so, which might make it
unsuitable for certain security-sensitive
applications.

uuid_generate_v1mc()

This function generates a version 1 UUID but uses
a random multicast MAC address instead of the real
MAC address of the computer.

uuid_generate_v3(namespace
uuid, name text)

This function generates a version 3 UUID in the
given namespace using the specified input name. The
namespace should be one of the special constants
produced by the uuid_ns_*() functions shown below.
(It could be any UUID in theory.) The name is an
identifier in the selected namespace.

uuid_generate_v4()

This function generates a version 4 UUID, which is
derived entirely from random numbers.

uuid_generate_v5(namespace
uuid, name text)

This function generates a version 5 UUID, which
works like a version 3 UUID except that SHA-1 is used
as a hashing method. Version 5 should be preferred
over version 3 because SHA-1 is thought to be more
secure than MD5.

For example:

SELECT uuid_generate_v3(uuid_ns_url(), 'http://www.postgresql.org');

The name parameter will be MD5-hashed, so the cleartext
cannot be derived from the generated UUID. The generation of
UUIDs by this method has no random or environment-dependent
element and is therefore reproducible.

Table F-34. Functions Returning UUID
Constants

uuid_nil()

A "nil" UUID constant,
which does not occur as a real UUID.

uuid_ns_dns()

Constant designating the DNS namespace for
UUIDs.

uuid_ns_url()

Constant designating the URL namespace for
UUIDs.

uuid_ns_oid()

Constant designating the ISO object identifier
(OID) namespace for UUIDs. (This pertains to ASN.1
OIDs, which are unrelated to the OIDs used in
PostgreSQL.)

uuid_ns_x500()

Constant designating the X.500 distinguished name
(DN) namespace for UUIDs.